1. Field of the Invention
This invention relates to thin film transistors using a semiconductor film of crystals that are collected having various azimuths (hereinafter referred to as crystalline semiconductor film) as represented by a polycrystalline silicon film, and a semiconductor device formed by using the above thin film transistors. In particular, the invention relates to a semiconductor film forming a channel-forming region, a source region and a drain region of a thin film transistor and to a semiconductor device mounting the above thin film transistors. In this specification, the semiconductor device refers to devices that work by utilizing semiconductor characteristics as a whole inclusive of display devices as represented by a liquid crystal display device and semiconductor integrated circuits (microprocessors, signal processing circuits and high-frequency circuits).
2. Prior Art
There has been developed a technology for fabricating thin film transistors (hereinafter abbreviated as TFTs) by forming a crystalline semiconductor film on a glass substrate or on a quartz substrate. Application of this technology has been forwarded in a field of flat panel displays as represented by an active matrix liquid crystal display device. TFTs are used as switching elements in the pixels or as elements for forming a driver circuit formed in the peripheries of the pixels.
Silicon is chiefly used as a material of a crystalline semiconductor film for forming the channel-forming regions, source regions, drain regions or low-concentration drain (lightly doped drain: LLD) regions in active regions of TFTs. The silicon film having a crystalline structure (hereinafter referred to as crystalline silicon film) is formed by subjecting an amorphous silicon film deposited on a substrate by a plasma CVD method or a low pressure CVD method to the heat treatment or to the irradiation with a laser beam (hereinafter referred to as laser treatment in this specification).
In conducting the heat treatment, however, the heating must be effected at a temperature of not lower than 600xc2x0 C. for not less than 10 hours to crystalize the amorphous silicon film. The above treating temperature and the treating time are not necessarily suitable from the standpoint of productivity of the TFTs. When a liquid crystal display device is taken into consideration as an applied product using TFTs, a heating furnace of a large size is necessary to cope with an increase in the area of the substrate, not only consuming energy in large amounts in the steps of production but also making it difficult to obtain homogeneous crystals over a wide area. In the case of the laser treatment, it is difficult to obtain homogeneous crystals due to the lack of stability in the output of the laser oscillator. Dispersion in the quality of crystals could become a cause of dispersion in the TFT characteristics, and deteriorates the quality of display of the liquid crystal display devices and the EL display devices.
There has also been proposed a technology for forming a crystalline silicon film through the heat treatment at a temperature lower than the temperatures employed thus far by introducing, into the amorphous silicon film, metal elements that assist the crystallization of silicon. According to, for example, Japanese Patent Application (Kokai) Nos. 7-130652 and 8-78329, a crystalline silicon film is obtained by the heat treatment conducted at 550xc2x0 C. for 4 hours by introducing such a metal element as nickel into the amorphous silicon film.
In the crystalline silicon film formed by the above conventional methods, however, the planes of crystalline azimuth exist in a random fashion, and the ratio of orientation is low for particular crystalline azimuths. The crystalline silicon film obtained by the heat treatment or the laser treatment permits plural crystalline particles to be precipitated and oriented on {111}. Even when limited to the plane azimuth, however, the ratio of orientation did not exceed 20% of the whole film.
When the ratio of orientation is low, it is almost impossible to maintain continuity of lattice on the crystalline grain boundaries where the crystals of different azimuths abut to each other, and it is estimated that unpaired bonds are formed much. The unpaired bonds on the grain boundaries could become centers of trapping the carriers (electrons/holes) accounting for a drop in the carrier transport property. That is, since the carriers are scattered and trapped, a TFT having a high electric-field mobility cannot be expected despite the TFT is fabricated by using the above crystalline semiconductor film. Besides, since the crystalline grain boundaries exist in a random fashion, it is difficult to form the channel-forming region using crystalline particles having a particular crystalline azimuth, and electric characteristics of the TFT tend to become dispersed.
It is an object of this invention to provide means for solving the above-mentioned problems, and to provide TFTs using a crystalline semiconductor film which is obtained by crystallizing an amorphous semiconductor film and is highly oriented, as well as to provide a semiconductor device mounting the above TFTs.
This invention provides a TFT having a channel-forming region formed of a crystalline semiconductor film obtained by heat-treating and crystallizing an amorphous semiconductor film containing silicon as a main component and germanium in an amount of not smaller than 0.1 atomic % but not larger than 10 atomic % (preferably, not smaller than 1 atomic % but not larger than 5 atomic %) while adding a metal element thereto, wherein an orientation ratio of the lattice plane {101} is not smaller than 20% and the lattice plane {101} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film, and an orientation ratio of the lattice plane {001} is not larger than 3% and the lattice plane {001} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film, and an orientation ratio of the lattice plane {001} is not larger than 5% and the lattice plane {111} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film as detected by the electron backscatter diffraction pattern method.
The invention further provides a TFT having a channel-forming region formed of a crystalline semiconductor film obtained by heat-treating and crystallizing an amorphous semiconductor film containing silicon as a main component and germanium in an amount of not smaller than 0.1 atomic % but not larger than 10 atomic % (preferably, not smaller than 1 atomic % but not larger than 5 atomic %) while adding a metal element thereto, wherein an orientation ratio of the lattice plane {101} is not smaller than 5% and the lattice plane {101} has an angle of not larger than 5 degrees with respect to the surface of the semiconductor film, an orientation ratio of the lattice plane {001} is not larger than 3% and the lattice plane {001} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film, and an orientation ratio of the lattice plane {001} is not larger than 5% and the lattice plane {111} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film as detected by the electron backscatter diffraction pattern method.
The invention further provides a TFT having a channel-forming region formed of a highly oriented crystalline semiconductor film having a thickness of from 20 nm to 100 nm and containing nitrogen and carbon at concentrations of smaller than 5xc3x971018/cm3, containing oxygen at a concentration of smaller than 1xc3x971019/cm3, and containing the metal element at a concentration of smaller than 1xc3x971017/cm3.
The invention further provides a semiconductor device having a channel-forming region formed of a semiconductor film obtained by heat-treating and crystallizing an amorphous semiconductor film containing silicon as a main component and germanium in an amount of not smaller than 0.1 atomic % but not larger than 10 atomic % (preferably, not smaller than 1 atomic % but not larger than 5 atomic %) while adding a metal element thereto, wherein an orientation ratio of the lattice plane {101} is not smaller than 20% and the lattice plane {101} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film, and an orientation ratio of the lattice plane {001} is not larger than 3% and the lattice plane {001} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film, and an orientation ratio of the lattice plane {001} is not larger than 5% and the lattice plane {111} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film as detected by the electron backscatter diffraction pattern method.
The invention further provides a semiconductor device having a channel-forming region formed of a semiconductor film obtained by heat-treating and crystallizing an amorphous semiconductor film containing silicon as a chief component and germanium in an amount of not smaller than 0.1 atomic % but not larger than 10 atomic % (preferably, not smaller than 1 atomic % but not larger than 5 atomic %) while adding a metal element thereto, wherein an orientation ratio of the lattice plane {101} is not smaller than 5% and the lattice plane {101} has an angle of not larger than 5 degrees with respect to the surface of the semiconductor film, an orientation ratio of the lattice plane {001} is not larger than 3% and the lattice plane {001} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film, and an orientation ratio of the lattice plane {001} is not larger than 5% and the lattice plane {111} has an angle of not larger than 10 degrees with respect to the surface of the semiconductor film as detected by the electron backscatter diffraction pattern method.
The invention further provides a semiconductor device having a channel-forming region formed of a highly oriented crystalline semiconductor film having a thickness of from 20 nm to 100 nm and containing nitrogen and carbon at concentrations of smaller than 5xc3x971018/cm3, containing oxygen at a concentration of smaller than 1xc3x971019/cm3, and containing the metal element at a concentration of smaller than 1xc3x971017/cm3.
The metal element that is added is one or more of those selected from Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au. The amorphous silicon film to which the metal element is added is heat-treated to thereby form a compound (silicide compound) of silicon with the metal element. This compound then diffuses to assist the crystallization. Germanium that is added to the amorphous silicon does not react with this compound but stays in the peripheries thereof to build up local strain. The strain works to increase the critical radius of the formation of nuclei and, hence, to decrease the density of the formation of nuclei. The strain further limits the orientation of crystals.
To produce the above-mentioned action, it has been learned through experiment that germanium needs to be added in an amount of not smaller than 0.1 atomic % but not larger than 10 atomic % (preferably, not smaller than 1 atomic % but not larger than 5 atomic %). When germanium is added in amounts larger than the above range, nuclei are formed spontaneously and conspicuously (crystalline nuclei that are not dependent upon the compound of the added metal element) as an alloy of silicon and germanium, making it difficult to increase the ratio of orientation of the obtained crystalline semiconductor film. When germanium is added in too small amounts, strain does not build up to a sufficient degree making it difficult to increase the ratio of orientation, either.
When the amorphous semiconductor film is crystallized, the volume of the film contracts due to the rearrangement of atoms if viewed macroscopically. As a result, tensile stress occurs in the crystalline semiconductor film formed on the substrate. Upon containing germanium having an atomic radius larger than that of silicon at a concentration of 0.1 to 10 atomic %, preferably, 1 to 3 atomic %, however, the contraction of volume due to the crystallization is suppressed, and a small internal stress occurs. That is, upon containing germanium at a concentration as contemplated by this invention, the strain in the crystalline semiconductor film can be relaxed.
The distribution of crystalline azimuths can be found by using an electron backscatter diffraction pattern (EBSP). The EBSP is a method of analyzing the crystalline azimuth from the backscattering of primary electrons by providing a scanning electron microscope (SEM) with a special detector (hereinafter, this method is referred to as EBSP method for convenience). FIG. 2 is a diagram illustrating the principle thereof. An electron gun (Schottky field-effect emission electron gun) 201, a mirror 202 and a sample chamber 203 are constituted in the same manner as those of an ordinary scanning electron microscope. To measure the EBSP, a stage 204 is tilted at an angle of about 60 degrees, and a sample 209 is installed. In this state, a screen 205 of a detector 206 is inserted so as to face the sample. Reference numeral 207 indicates an electron beam; 208, a backscattered electron.
Here, when an electron ray falls on the sample having a crystalline structure, non-elastic scattering also takes place on the back side thereof, and there can be also observed a linear pattern (generally called Kikuchi image) specific to the crystalline azimuth due to Bragg diffraction in the sample. According to the EBSP method, the Kikuchi image reflected on the detector screen is analyzed to find the crystalline azimuth of the sample.
FIG. 3 illustrates a crystalline semiconductor film 302 of a polycrystalline structure formed on a substrate 301. The crystalline semiconductor film 302 has a prerequisite in that each crystalline particle therein has a different crystalline azimuth. Upon repeating (mapping) the azimuthal analysis while moving a position of the sample where the electron beam falls, the data related to the crystalline azimuth or to the orientation can be obtained concerning the planar sample. The thickness of the incident electron beam 303 varies depending upon the type of the electron gun of the scanning electron microscope. In the case of the Schottky electric-field emission electron gun, an electron beam of as very fine as 10 to 20 nm can be projected. In the mapping, more highly averaged data of crystal orientation are obtained with an increase in the number of the measuring points or with an increase in the area of the measured region. In practice, about 10000 points (a gap of 1 xcexcm) to about 40000 points (0.5 xcexcm) are measured over a region of 100xc3x97100 xcexcm2. Reference numeral 304 indicates a backscattered electron.
When the crystalline azimuths of the crystalline particles are all found by mapping, the state of crystal orientation for the film can be expressed in a statistic manner. FIG. 4A is a diagram illustrating back poles found by the EBSP method. The diagram of the back poles is frequently used for displaying the preferential orientation of a polycrystalline substance and collectively represents which lattice plane a particular plane (surface of the film, here) of the sample is in agreement with.
A fan-shaped frame of FIG. 4A is usually called a standard triangle in which are included all indexes of the cubic crystal system. The length in this diagram corresponds to an angle in the crystalline azimuth. For example, an angle of 45 degrees is defined by {001} and {101}, an angle of 35.26 degrees is defined by {101} and {111}, and an angle of 54.74 degrees is defined by {111} and {001}. White dotted lines represent ranges of shearing angles of 5 degrees and 10 degrees from {101}.
FIG. 4A is the one in which all measuring points (11655 points in this example) in the mapping are plotted within the standard triangle. It will be learned that the density is high near the point {101}. FIG. 4B shows the concentration of such points using contour lines. These are the values of an azimuth distribution function, and the concentration (density of points of FIG. 4A) is represented by a contour line in the case when a random orientation is presumed. Here, the values represent magnifications of when it is presumed that the crystalline particles are oriented in a quite orderless manner, i.e., when the points are evenly distributed in the standard triangle, and are the values without dimension.
When it is learned that the crystalline particles are preferentially oriented to a particular index (here, {101}), the ratio of the number of crystalline particles collected near the index is indicated by a numerical value, so that the degree of preferential orientation can be easily imagined. In the diagram of back poles shown in FIG. 4A, for example, the ratio of the number of points present in a range between a shearing angle of 5 degrees and a shearing angle of 10 degrees from {101} (indicated by white dotted lines in the drawing) to the total number of the points can be expressed as a ratio of orientation in compliance with the following formula.
[Formula 1]
{101} Ratio of orientation=number of measured points within an allowable angle between the lattice plane {101} and the film surface/total number of the measured points
This ratio can be explained in a manner as described below. When the distribution is concentrated near {101} as in FIG. 4A, the individual particles in a real film have an azimuth  less than 101  greater than  nearly perpendicular to the substrate as shown in FIG. 6 but are expected to be arranged being fluctuated thereabout. The allowable values of the angle of fluctuation are set to be 5 degrees and 10 degrees, and the ratio of those smaller than these values are numerically expressed. Reference numeral 601 indicates a substrate; 602, a crystalline semiconductor film. In FIG. 5, for example, the azimuth  less than 101 greater than 505 of a given crystalline particle is not included in an allowable range of 5 degrees 503 but is included in an allowable range of 10 degrees 504. In the data appearing later, the allowable shearing angles are set to be 5 degrees and 10 degrees as described above, and the ratio of crystalline particles satisfying this is expressed. Reference numeral 501 indicates surface of a film; 502, a perpendicular line of surface.
In the diagram of back poles shown in FIG. 4A, the vertexes are {101}, {111} and {001}, and the other plane azimuth appears as the shearing value increases relative to {101}. As the shearing angle from {101} becomes 30 degrees, then, {112} develops. When the ratio of existence of crystalline azimuth is to be determined by the EBSP, therefore, the allowable shearing angle must be determined for the crystalline particles that are distributed in a fluctuated manner so as not to include other indexes. The present inventors have discovered that the ratio of existence of crystalline particles oriented in a particular azimuth can be quantitatively expressed by collecting the data while setting the allowable shearing angle to be smaller than 10 degrees or smaller than 5 degrees.